Laser ultrasonics for non-contact materials characterization of fiber reinforced materials

The optical generation of ultrasound is based at low power levels on the thermoelastic effect, which is the generation of elastic waves by transient surface heating with a laser pulse. At high power level the laser source operates in the ablation regime by vaporizing a small amount of surface material. The radiated field of such a source resembles a monopole radiating in all directions of the half space. Laser ultrasonics uses one laser with a short pulse for the generation of elastic waves and another one, a long pulse or continuous laser, coupled to an optical interferometer for the detection of ultrasound. This technique enables a completely contactless measurement of elastic constants. As the laser pulse generates bulk as well as surface acoustic waves (SAW) both can be used for non-contact material characterization. Short laser pulses were used to generate bulk waves in continuous fiber reinforced aluminium or magnesium. With an optical interferometer the run-time of these waves in various directions of the samples can be measured in a contactless way (point-source-point-receiver technique). From these data, the five independent components of the complete elastic tensor for the unidirectional fiber reinforced composites can be determined. These results agree with measurements by a resonant beam technique, acoustic resonance and 4-point bending tests. For the generation of SAWs the pulsed laser was focused by a cylindrical lens to a thin line to concentrate the acoustic energy in a direction perpendicular to the line. Material properties of anisotropic films deposited on a silicon wafer were determined by phase velocity dispersion measurement. The acoustic waves were detected by two optical methods: a confocal Fabry Perot Interferometer and a beam deflection setup. The laser pulse technique has several advantages: it is contactless, non-destructive and it can be used for online quality control in production processes.